Combining .exr files?
Yes you can load several EXRs on the compositor and save all of their channels into individual layers on an OpenEXR multilayer.

Add a file output node, set it to OpenEXR muyltilayer and than add as many inputs as you need. 



You can rename the channels as needed by double clicking on the name:



For more details see this answers:

https://blender.stackexchange.com/a/18406/1853

How can I render all render passes to files in Cycles?